Download Skeletal Reflexes - University of Houston College of Optometry

Skeletal Reflexes
Lanny Shulman, O.D., Ph.D.
University of Houston
College of Optometry
 Afferent
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10 million
Connect sensory receptor in PNS with spinal cord or
brain
Receptors can be grouped into 4 categories:
1. somatic sensory receptors
2. external receptors
3. proprioceptors
4. visceral or internal receptors
 Efferent
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System: Sensory Neurons
System: Motor Neurons
0.5 million
Carries instructions from the CNS to other tissues,
organs
Motor System
 Refers
to neural pathways that control
contractions of the skeletal muscles
 Skeletal muscle contractions result in:
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Posture
Reflexes
Rhythmic Activity (e.g. locomotion,
respiration)
Voluntary movements
Corticospinal Pathway
• Conscious
control of skeletal
muscles throughout the
body
Other Pathways:
Medial and Lateral Pathways
Subconscious regulation of
skeletal muscle tone, controls
reflexive skeletal muscle
responses to equilibrium
sensations and to sudden or
strong visual and auditory
stimuli
(Dorsal horn)
(Ventral horn)
Sensory System
 Afferent
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Pathways
Sensory
Posterior Column Pathway
Spinothalamic Pathway
Spinocerebellar Pathway
Posterior Column Pathway
•Fine touch, pressure, vibration,
and proprioception
Other Pathways:
• Spinothalamic Pathway
touch, pressure, pain,
temperature
• Spinocerebellar Pathway
proprioceptive information
Muscle Fibers and Innervation
 Extrafusal
Fibers: contractile muscle fibers
 Intrafusal Fibers: sensory muscle fibers
(muscle spindle)
Muscle Spindle
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Nuclear Bag Fibers: central nuclear area is
dilated
 Nuclear Chain Fibers: No dilation; nuclei
arranged in single row
 Report Muscle Length
Golgi Tendon Organs
• Found in tendons, connective tissue, around joint capsules
• Supplied by Group 1B fibers
• Report muscle tension
Simple Reflexes
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Sensory fibers deliver information to CNS
 Motor fibers carry motor commands to
peripheral effectors
 “Wiring” of a single reflex: REFLEX ARC
Reflex Arc
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Sensory information is processed at the level of the
spinal cord
Resulting in action without involvement of the brain
If the hand is burned by a candle, the hand is rapidly
removed from the flame
At the same time, the brain is receiving information
that a painful event occurred in the hand
Although the brain may perceive the painful stimulus,
the reflex arc acts to retract the hand without direct
input from the brain.
The best known example of a reflex arc is the Stretch
Reflex.
Stretch Reflex
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Stretch reflex provides automatic regulation of
skeletal muscle length.
The sensory receptors in the stretch reflex are
muscle spindles.
Stretch reflexes are important in maintaining
normal posture and balance.
Makes automatic adjustments in muscle tone.
Stimulus is increasing muscle length
Activates a sensory neuron that triggers a motor
response which is contraction of the stretched
muscle. This counteracts the stimulus.
Can use stretch reflex to test general condition
of the spinal cord, nerves and muscle
The Stretch Reflex
Withdrawal Reflex or Flexor Reflex
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Have at least one interneuron
Therefore, they are polysynaptic reflexes
Designed to avoid harmful stimuli
Move body parts away from the source of
stimulation
Strongest reflexes are triggered by pain (also touch
or pressure)
These are ipsilateral reflexes (same muscle
contracts on the same side as the stimulus)
These are intersegmental reflexes (stimulation of
motor neurons come from different segments of the
spinal cord)
Withdrawal Reflex
Tendon Reflex or the Inverse
Stretch Reflex
• Opposite of the Stretch Reflex
• Instead of responding to excessive stretching, this
reflex responds to excessive contraction.
• The tendon reflex is a protective feedback
mechanism to control the tension of a muscle by
causing relaxation.
• The stretch reflex is a feedback mechanism to control
muscle length by causing muscle contraction.
• The tendon reflex can over-ride the stretch reflex
when tension is too great.
• Affects rate of discharge of motor neuron
Tendon Reflex
Spinal Cord Deficits
• Babinski’s Sign:
• Damage to lateral corticospinal tract
• Withdrawal reflex that is held in check by the lateral
corticospinal system
Sensory Endings in Extraocular
Muscles and Tendons
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Human extraocular muscles have anatomically
degenerated sensory organs (muscle spindles
and Golgi tendon organs)
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Small
Capsules poorly defined
Fewer intrafusal fibers
No obvious distinction between bag and chain fibers
Human extraocular muscles have no stretch
reflexes
What is the role of sensory
organs in extraocular muscle?
 No
large or sudden change in load in
extraocular muscles
 no reason for having mechanism to
compensate for loading changes
 Could explain the degeneracy of sensory
organs in number and structure
 They’re present because they are a
natural constituent of striated muscle
Oculocardiac Reflex
 Passive
stretch could cause bradycardia
 Anatomical pathway is unknown
 No functional role for the reflex
 Probably aberrant neural wiring in the
brainstem
 Presumably a developmental anomaly in
some individuals
Sensory endings in extraocular
muscles probably do not provide
information about eye position
 Experiments
show that sensory endings of
extraocular muscles only provide a small
effect on perception of visual direction and
ocular alignment.
Why are there sensory signals?
 Sensory
signals may be involved in motor
learning, motor plasticity, and
development
 This role could be important during growth
and development
 Is the anatomical degeneracy related to
age?